Sensors have been used to measure flow rates in various medical, process, and industrial applications, ranging from portable ventilators supplying anesthetizing agents to large-scale processing plants in a chemical plant. In these applications, flow control is an inherent aspect of proper operation, which is achieved in part by using flow sensors to measure the flow rate of a fluid within the flow system. In many flow systems, e.g., fuel cell flow systems containing a binary mixture of methanol and water, the chemical composition of the fluid may change frequently.
A flow system is often required to flow more than one fluid having different chemical and thermo physical properties. For example, in a semiconductor processing system that passes a nitrogen-based gas, the nitrogen-based gas may at times be replaced by a hydrogen-based or helium-based gas, depending on the needs of the process; or in a natural gas metering system, the composition of the natural gas may change due to non-uniform concentration profiles of the gas.
Fluid flow sensors are thus important in a variety of applications. It is often necessary to determine the composition of a fluid utilizing a liquid or fluid flow sensor. One method for determining the composition of the fluid is to measure its thermal conductivity and compare the resulting value to a standard value. Measurements can be obtained by measuring power transferred from a heater to the fluid. In many cases, the fluid should not come into contact with the sensor and/or associated heater due to material incompatibility, explosion proof applications, or even medical hazards. A compatible material should therefore be placed between the fluid and the sensor and/or heater. Such material, however, typically dissipates power away from the fluid and the sensor, thereby reducing the thermal efficiency and therefore the signal quality. What is needed, therefore, is an enhanced sensor configuration that can overcome the aforementioned drawbacks.
One example of a flow sensor is disclosed in U.S. Pat. No. 6,871,537, entitled “Liquid Flow Sensor Thermal Interface Methods and Systems,” which issued to Richard Gehman, et al. on Mar. 29, 2005. U.S. Pat. No. 6,871,537, which is assigned to Honeywell International Inc. and is incorporated by reference herein, generally describes a sensor method and system in which a fluid flow sensor is provided that measures the thermal conductivity of a fluid. The sensor is configured to include one or more sensing elements associated with a sensor substrate.
As described in U.S. Pat. No. 6,871,537, a heater is associated with the sensor and provides heat to the fluid. A film component can also be provided that isolates the fluid from the heater and the sensor, such that the film component conducts heat in a direction from the heater to the sensor, thereby forming a thermal coupling between the sensor, the heater and the fluid, which permits the sensor to determine a composition of said fluid by measuring thermal conductivity thereof without undesired losses of heat in other directions. The film component is generally configured on or in the shape of a tubing or a flow channel.
Airflow sensing chips have been utilized in a number of sensing applications, and can include the use of a physical bridge, approximately 1 micrometer thick, that thermally isolates sense resistors, ambient temperature sensors and heater resistors from each other, which can form a part of the airflow sensing chip configuration. Such devices function very well in air flow applications. The use of such a thin bridge, however, is inherently fragile and if exposed to liquid flow results in damage to the bridge, in effect a “wash out,” which effectively destroys the sensing capability of the airflow sensing chip.
In order to sense liquid flow, chips of this type can be “ruggedized” by eliminating the physical bridge. In such a situation, however, a different substrate is required because standard silicon has a very high thermal conductivity and the resistors associated with the sensor chip tend to operate toward the same temperature, which prevents proper sensing functions. Substrates other than silicon may be utilized, which provide low thermal conductivity and are compatible with wafer processing. One such substrate material is optoelectronic grade quartz, assuming that that Pt resistors are being utilized in association with the sensor chip. An alternate approach which has been successfully demonstrated involves the use of Nickel Iron alloys on a substrate of Pyrex glass. Pyrex has a much lower thermal conductivity than quartz and provides double the sensor output and lower errors. Nickel-iron may also be utilized, but tends to corrode easily. Processing Pt on Pyrex generally does not work well, because the Pyrex melts at the Pt anneal temperature.
Constructing a liquid flow sensor on solid quartz is possible, but its production has been limited. Compared to other airflow sensors, however, the resulting product has low sensitivity, far higher drift, very large temperature errors, excessive power dissipation, and a very long warm-up time. Most of these problems are due to the thermal cross talk (inside the quartz) between those resistors which, on an airflow sensor, are thermally isolated (by air gaps). It is believed that it may be possible to recover from some of those errors by physically separating the cross-talking resistors.
Thermal flow sensors typically require controlled heat sinking to direct heat flows between the media and the heating and sensing functions. It is often necessary, however, to minimize heat that flows in other directions, which lowers thermal efficiency, distorts output signals, increases response time and warm-up time, causes output drift/noise and requires excessive power to operate. It is believed that a solution to these problems involves the design and implementation of an improved flow sensor system, which is described in greater detail herein.